Bioelectrical impedance analysis (BIA) is a commonly-used, non-invasive technique for estimating the composition of the body of a human or animal. It has been practiced in whole body and segmental formats. In broad outline, current is applied to the body between at least two spatially-separated points (the current application points) and the voltage difference produced by the applied current is measured between at least two other spatially-separated points (the measurement points). Typically, the measurement points are located inboard of the current application points. Measurements can be performed at a single frequency or at a series of frequency, in which case the technique is sometimes referred to as BIA spectroscopy.
The impedance Z is determined by taking the ratio of the measured voltage V divided by the applied current I, where Z, V, and I are, in general, complex numbers. Although the impedance Z can be of value for some applications, normally, it is desirable to normalize the impedance (or one of its components) by the physical dimensions of the portion of the body over which the measurement was taken. For example, it is often desirable to derive a resistivity (ρ) value from a resistance (R) value using the equation ρ=R·A/L, where L is length and A is cross-sectional area, e.g., A=C2/4π for a circular cross-section whose circumferential length is C.
Of the two dimensions L and A, L is normally easier to estimate. Thus, L can be well-approximated by the linear distance between the spatially-separated measurement points. Estimating A, on the other hand, is more difficult for the fundamental reason that body tissues are compressible.
Although health care and other professionals (e.g., weight loss coaches, physical trainers, and the like) can be taught to measure the circumference of a portion of the body with a tape measure, the measurement requires judgment as to how tight to make the tape. The need for judgment results in substantial and unacceptable variability between measurements made by different professionals, as well as in measurements made by the same professional with different individuals or the same individual on different occasions. For lay personal, the problem is markedly worse. Moreover, other than for measurements on the legs, circumference measurements are difficult for an individual to do on himself or herself, e.g., it is difficult to apply a tape measure to one's own arm. Even leg measurements can be difficult for some individuals whose eyesight and/or dexterity has been compromised.
The present disclosure addresses this problem of unreliable circumference measurement which has reduced the usefulness of BIA and, in particular, segmental BIA, both in clinical and at-home settings.